The present invention relates to an electrophotographic type image forming device used in laser printers, photocopiers, facsimile machines, and the like.
A conventional electrophotographic type image forming device, such as a laser printer, has been disclosed, for example, in laid open Japanese patent application publication No. HEI-10-35941. In this type of image forming device, a toner image formed on a photosensitive drum is transferred to a sheet or another recording medium as the medium passes between a transfer roller and the photosensitive drum. Subsequently, the recording medium passes between a single heat roller and a single pressure roller, causing the toner to melt by heat and fixing the toner image to the surface of the recording medium. A sheet supply unit is provided for supplying the recording medium stacked in a sheet supply tray, one sheet at a time. The sheet of the recording medium is conveyed from the sheet supply unit to an image forming section at which the photosensitive drum opposes the transfer roller through a conveying path by a sheet supply roller, a plurality of intermediate conveying rollers, and a register roller provided along the conveying path. The register roller is intermittently driven while the plurality of intermediate conveying rollers are rotated. A leading edge of the image recording medium abuts against the stopped register roller while the medium is fed by the conveying rollers, so that an intermediate area of the recording medium can be flexed. As a result, the leading edge can be aligned with a proper orientation, while the supply timing of the medium to the image forming section can be adjusted.
When forming images on a plurality of recording mediums in succession, the start timing for supplying the second and subsequent sheets from the sheet supply unit is set as follows. If the trailing edge of the preceding sheet is too close to the leading edge of the following sheet, a leading edge of a subsequent sheet may reach a sheet sensor disposed along the conveying path from the sheet supply unit to a discharging unit prior to the detection of completion of passing of the precedent sheet through the sheet sensor due to unevenness of leading edge positions of the sheets in the sheet supply tray. In the latter case, judgment is made such that the precedent sheet is not subject to feeding but staying at the sheet sensor position, and accordingly, an error processing attendant to sheet jamming is improperly executed.
If the subsequent sheet is fed prior to the separation of the trailing edge of the precedent sheet from the sheet supply roller, unwanted force is applied to the precedent sheet, which degrades sheet conveying performance. To avoid this problem, normally, the start timing for supplying sheets is set such that an interval of a prescribed amount, for example approximately 60 mm, is provided between the trailing edge of the preceding sheet and the leading edge of the following sheet taking also image data developing period into consideration.
For supplying each one sheet from the sheet supply unit, a frictional separating mechanism is used for separating the one sheet from the remaining sheet stack. One such frictional separating mechanism employs a separation pad pressing against the sheet supply roller to generate friction for separating the sheets. However, sometimes the leading edges of the sheets in the sheet supply unit become uneven. When using such frictional separating mechanism, a following sheet can oppose the frictional force between the sheet supply roller and frictional separating pad and move in the conveying direction as the preceding sheet is conveyed due to attractive force between the preceding uppermost sheet and the subsequent lower sheet stacked in the sheet supply tray. In this case, the subsequent sheet stops with its leading edge shifted downstream in the conveying direction from a reference position at the nip area between the sheet supply roller and frictional separating pad when the sheet supply roller has stopped.
If the following sheet stops with its leading edge shifted too far downstream from the reference point at the nip area when the sheet supply roller stops, the required condition of maintaining a prescribed interval between neighboring sheets cannot be met. Hence, when the sheet supply roller is driven to rotate at a prescribed timing designed to maintain a prescribed interval between sheets, it is possible that the sheet sensors will detect a sheet jam or that sufficient time is not allocated for developing the image data to be printed.